1. Field of the Invention
The invention relates to a magnetic resonance spectrometer in which at least two mixing stages are provided in the receiving branch in order to transpose the spin resonance signals into a lower frequency range, and in which an analog-to-digital converter converts the spin resonance signals thus transposed into the lower frequency range into a series of digital data words, the sampling frequency being at least twice as high as the highest frequency in the transposed spin resonance signal.
2. Description of the Related Art
A spectrometer of this kind is known from West German patent DE-OS 2 928 551 corresponding to U.S. Pat. No. 4,297,637, notably from FIG. 13. During each mixing operation in such a spectrometer, not only the signals from the useful frequency range are transposed into the frequency range of the output signal, but also signals from the mirror frequency range. (As is known, the mirror frequency is that frequency which is situated at the same frequency distance as the useful frequency from the mixing frequency with which the useful frequency is mixed). The noise and interference signals in the mirror frequency range, therefore, must be suppressed by means of a filter arranged in the signal path preceding the mixing stage. The requirements imposed on this filter are comparatively low in the known spectrometer, because the first frequency with which the spin resonance signal is mixed is substantially higher than the Larmor frequency, so that the mirror frequency range can be suppressed by means of a filter having a slope which is not very steep.
In the second mixing stage, the output signal of the first mixing stage is mixed down to the baseband. To this end, the second mixing stage is constructed as a quadrature mixer in which the output signal of the first mixing stage is multiplied by two signals which have been shifted through 90.degree. with respect to one another and whose frequency corresponds exactly to the mean value in time of the output signal of the first mixing stage. As a result of this mixing operation it is avoided that interference or noise situated outside the frequency range of the spin resonance signal is transposed into the base band.
However, this advantage is obtained at the expense of a number of drawbacks. The quadrature mixer must have an exactly symmetrical construction and the phase shift between the two sinusoidal signals must amount to exactly 90.degree.. It is comparatively complex to satisfy these requirements in an analog mixing stage. A further drawback consists in that the lowest useful frequency in the baseband is so low that, a d.c. amplifier must be used to amplify the output signal of the quadrature mixer. Shifts of the operating point of this amplifier causes a disturbing drift which causes reconstruction errors during the further processing of the signal. Moreover, the low-frequency 1/f noise as well as harmonics of the mains frequency have a particularly disturbing effect in the baseband.